1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a charged photoconductor (for example, a photoconductor drum) with (to) laser light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner from a developing device to the photoconductor on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to a sheet through an intermediate transfer belt, followed by heating and pressurization for fixing, whereby an image is formed on the sheet.
The above-described image forming apparatus includes a sheet feed tray, a sheet conveyance section that conveys to an image forming section a sheet fed from a manual feed tray or an external sheet feeding device. In the sheet conveyance section, a plurality of conveyance rollers such as intermediate conveyance rollers, loop rollers, and registration rollers are disposed, for example.
When a sheet is conveyed by the conveyance section, the sheet may be displaced in a lateral direction of the sheet (a horizontal scanning direction, or a direction orthogonal to a sheet conveyance direction). The cause of this displacement includes, for example, an axially nonuniform roller diameter due to errors in manufacture, a variation in roller diameter due to aging degradation, a displaced sheet stored in the sheet feed tray, and the like. When an image formation is performed with the laterally displaced sheet described above, the image forming region on the sheet is deviated.
To solve such a problem, a method for correcting the displacement by using a registration translation has been proposed as a method for accurately aligning an image with a sheet in consideration of the lateral displacement of the sheet (see, for example, Japanese Patent Application Laid-Open Nos. 2007-22680 and 2013-6643). To be more specific, a displacement of a sheet is corrected in such a manner that registration rollers tightly sandwiching the sheet conveys the sheet while translating in the lateral direction of the sheet (the axis direction of the registration rollers).
This registration translation operation is performed based on detection results (a displacement amount and deviation from a reference position) obtained by displacement detection sensor 85 (a line sensor for example) disposed on the downstream side of registration rollers R in the sheet conveyance direction (see FIG. 1). As illustrated in FIG. 1, when the position of a sheet which is not laterally displaced is set as a reference position and the detection value obtained by displacement detection sensor 85 at this time is represented by detection reference value X0, displacement amount ΔX of a conveyed sheet is expressed by detection reference value X0− detection value Xi. In FIG. 1, the position of the conveyed sheet is displaced to − side (right side in FIG. 1) relative to the reference position by ΔX. Displacement amount ΔX is a translation amount (hereinafter referred to as “required translation amount”) required to put the sheet back to the reference position.
In this case, registration rollers R are translated to + side (left side in FIG. 1) so as to align the left end of the sheet in the lateral direction with an end of the reference position. At this time, when the sheet has a high translation responsiveness (followability) to registration rollers R and the sheet is translated by the same amount as registration rollers R, it is only necessary to translate registration rollers R to the left side by displacement amount ΔX.
However, in practice, the translation amount of registration rollers R based on a translation command value and the actual translation amount (measured value) of a sheet do not match. The translation responsiveness is varied by a conveyance path on the upstream side of the registration roller section in the sheet conveyance direction, looseness of a driving mechanism, and/or a load during driving. For example, in FIG. 2, inclinations a1 and a2 of lines L representing the translation responsiveness depend on the sheet type and the conveyance path, and intercepts b1 and b2 depend on the looseness of the driving mechanism and the load during driving. Here, the “translation command value” is information (pulse signal) which is input to a drive motor of registration rollers R so as to translate registration rollers R by a predetermined translation amount. The terms “translation command value” and “registration amount of registration rollers” used herein have the same meaning.
Generally, as illustrated in FIG. 2, the translation responsiveness is such that the translation amount of a sheet is smaller than that of the registration rollers. For example, according to the translation responsiveness in FIG. 2, when the translation command value is set at “+4 mm” and the registration rollers are translated by +4 mm, the actual translation amount of a sheet is “+3 mm.”
Under such circumstances, in the conventional image forming apparatus, a look-up table (hereinafter referred to as “translation control table”) indicating the relationship between a required translation amount and a translation command value (translation amount of the registration rollers) is prepared, and the translation command value is determined based on the translation control table, thereby translating a sheet by the desired amount. As illustrated in FIG. 3, the translation control table is defined by translation control line M expressed by an inverse function of line L representing the translation responsiveness. According to the translation control table defined by translation control line M, when the required translation amount is “3 mm to the + side,” the translation command value is “4 mm to + side,” for example.
Translation control line M is obtained in such a manner that, at the manufacturing stage of an image forming apparatus, a plurality of sheets are actually conveyed while being laterally displaced and the actual translation amount (measured value) of the sheets in the case where the translation command value is set in accordance with the displaced amount is measured to compute translation response line L, for example.
Incidentally, when registration rollers and/or bearings of the registration rollers are degraded due to abrasion, the nip pressure of the registration rollers decreases, and/or the looseness of the driving mechanism increases, and thus, the translation responsiveness decreases with time (see FIG. 2). However, the translation control table is set at the manufacturing stage of the image forming apparatus, and the variation of the translation responsiveness with time is not taken into consideration. That is, when the translation responsiveness is varied due to the operating condition (such as the number of printed sheets) of the image forming apparatus and the like, sheets may not be translated by the required translation amount even when the registration rollers are translated in accordance with the initially set translation control table, and thus the displacement correction may not be appropriately performed.
Conventionally, the components of a registration translation mechanism including registration rollers and bearings of the registration rollers are replaced according to a running condition set in advance (for example, the total number of printed sheets is 1,000,000) to deal with variation of the translation responsiveness with time. However, depending on the use environment, the components of the registration translation mechanism are replaced even when they are still usable, thus resulting in increase in cost per print (CPP).